In the mobile phone or the digital audio player, the use of class D amplifiers, which are excellent in electric power efficiency and can be miniaturized, is increasing.
In the integrated circuit class D amplifier, a DC offset is generated which is influenced by variations resulting from the manufacturing process, power supply voltage, temperature conditions to be used, aging, etc. Moreover, in a class D amplifier, although a Pulse-Width-Modulation signal (henceforth, PWM signal) is generated by calculating the difference between the input signal and a sawtooth signal or a triangular wave signal, the duty cycle of a PWM signal is also affected by the influence of the power supply voltage being used, temperature conditions, aging of the device, etc.
Although change of the DC offset by device variations can be reduced using trimming, changes in the DC offset and duty cycle caused by power supply voltage, temperature conditions, and aging cannot be reduced only with trimming technology, a thus a calibration circuit is used and the calibration of the DC offset and duty is periodically reset.
However, even if the calibration circuit is able to compensate for the DC offset and duty cycle variation which occur with the class D amplifier, it is common to prepare a deadtime generation circuit on the back-end side of the calibration circuit in order to prevent a flow-through current. The conventional calibration circuit is not able to compensate for this DC offset and duty change that arises using this dead-time generation circuit. Moreover, when a calibration circuit is prepared, there is a possibility that the scale of the circuit of a class D amplifier may become quite large depending on the circuit configuration.